Just, for the sake of argument, taking Khandro's "vulnerable atheist" hypothesis to be true, why might it be? In this country, at least, it's relatively less of a problem, but in the Western World it's not uncommon for "atheist" to be a very insulting label indeed. Not one week ago I had an argument with an American friend of mine about Bernie Sanders, who is someone he could not possibly vote for because not only is he socialist (and therefore the first step on the road to a communist dictatorship), but also, terrifyingly, atheist. It was not easy to work out which was the worse accusation.
I suppose in a roundabout way the point I'm trying to make is that in a society that is still predominantly religious, having any religion at all can often be seen as better than having no religion. This has a tendency to make atheists outcasts. In my personal experience, this was probably a self-imposed outcast status (not exactly helped, perhaps, by being at a CofE school, although I don't recall any teachers having an issue with it); for others, particularly in the US, isn't it reasonable to think that people who either feel rejected by their family, or actually are, would be that bit more vulnerable to being exploited by anyone that can give them a sense of belonging?
The main point then is that atheism is not in itself responsible for any increased risk of vulnerability. How people perceive atheists might well be, though.